In offshore seismic operations, an umbilical cable is required to pull a gun array, as well as to provide air, power and electrical conductors for shooting operations. Conventional practice in this art has been to use jacketed bundles which contain various air hoses, tension cables and electrical conductors or to use armored cables containing hoses and conductors. Such bundles do not last long because tow forces, wave forces and cable handling loads reduce the structural integrity of the umbilical cable to a point where conductors break and leak. The tension cables tend to abrade the electrical conductors, particularly when the bundle is reeled around a sheave or a drum under tension. More specifically, the tension cables tend to put point pressures on the electrical conductors, causing breakage and insulation leakage. This problem has been recognized heretofore, and one solution has been to use a discrete wire rope tension cable as a "clothesline" from which to intermittently tie a round jacketed bundle of electrical cables and air hoses. Thus, the wire rope cable provides the tensile strength, and the electrical/air bundle adjacent to it is not significantly loaded in tension. This method has worked reasonably well as long as the bundle can be drawn up "accordian style" without reeling it up. But, as longer and longer cables are needed for towing gun subarrays further outboard of the tow vessel, the compacted clothesline bundle is too long and causes too much drag to be effective and practical. Another problem with this method is that very short-radius bends form in the bundle, and as the cycles of the bends increase, the bundle life is decreased.
An alternative to the cable/bundle system is to use an umbilical cable with tension wires, conductors and air hoses cabled into a single "cable". The problem with this is that cyclically bending these cables around sheaves causes the wires to crush the conductors and hoses reducing cable life due to leakage.
Still another alternative is to build an armored cable with an outer-shell tension member, and hoses and electrical conductors within. This is feasible from a strength standpoint and is reelable but has several problems: first, the umbilical cable is excessively heavy; second, the terminations are difficult to seal; and third, the cables are expensive to replace and have questionable reliability.
Still another serious problem with all of the above-mentioned umbilical cable designs is that they tend to have a large overall diameter as well as a poor cross-sectional shape, thus causing high drag forces. The problem with high drag has come about because of increasing requirements to tow guns in a wide array and at higher speeds as shown in FIG. 1, and more particularly discussed hereinafter, as contrasted with narrower widths used previously.
An umbilical referred to as Flexpak.TM. is manufactured by Hydril Corporation (Bulletin 5086). The Flexpak.TM. umbilical tends to "cup" into flow inasmuch as it utilizes tensioning cables at both extremities and is not the equivalent of the present invention.
An umbilical with a faired shape referred to as Flexnose.RTM. is manufactured by Fathom Oceanology Ltd. (brochures MSK 4, September 1976 and MSK 61, August 1976). The Flexnose.RTM. is a preformed clip-on or clip-together and is not equivalent to the integrally molded faired umbilical of the present invention.